This invention relates to a method of manufacturing an oxygen sensing element capable of measuring partial pressures of oxygen in sample gases. The oxygen sensor device is suitable for use in detecting the concentration of oxygen in an exhaust gas from an internal combustion engine of an automobile. It is particularly suitable for use in an exhaust gas purifying system wherein the concentration of oxygen in the exhaust gas is measured, thereby to determine the contents of unburnt hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas, and based on the measurement results, the air-fuel ratio is appropriately adjusted so that the efficiency of a catalyst for purifying the exhaust gas is enhanced.
An oxygen sensor is an oxygen concentration cell which has electrodes mounted on the opposite sides of a solid electrolyte composed of a sintered ceramic material capable of conducting an oxygen ion, such as, for example, zirconia stabilized with a minor proportion of Y.sub.2 O.sub.3, CaO or MgO. An electromotive force is produced across the solid electrolyte by the difference between the partial pressures of oxygen in a reference gas and an exhaust gas, contacting opposite sides of the solid electrolyte. The concentration of oxygen in the exhaust gas can be determined by measuring the electromotive force as produced. Conventionally, air is used as the reference gas. The reference gas may also be generated chemically by using a mixture of a metal and its oxide (for example, a mixture of iron and iron oxide) which produces an equilibrium partial pressure of oxygen. Such a metal-metal oxide mixture is hereinafter referred to as "reference solid electrode" for brevity.
An oxygen sensing cell or element has a structure of the type wherein air is introduced or a reference solid electrode is enclosed in a solid electrolyte member of a tubular form, one end of which is closed, or a cup form; an electrode layer composed of an electrochemically active metal, such as platinum, is provided on the interface between the reference solid electrode or air and the solid electrolyte member, and; an external conductive metal electrode or electrodes to be exposed to the exhaust gas are mounted on the exterior surface of the solid electrolyte member. However, this oxygen sensing element is not advantageous for the following reasons. First, it is difficult or complicated to form a uniform electrode layer on the interface between the reference solid electrode or air and the solid electrolyte member, and thus, it is difficult to avoid variability in some performances, such as the operating temperature, the response time and the internal resistance among the resulting sensing elements. Secondly, it is also difficult to form a durable seal in the upper opening of the cup-shaped or tubular sensing element, and thus, it is difficult to maintain a constant partial pressure of oxygen generated by the reference solid electrode over a long period of time.
Another oxygen sensing element is of a short columnar shape and has a structure such that a reference solid electrode is completely embedded or enclosed within a solid electrolyte member, and; two external conductive metal electrodes are mounted on the exterior surface of the solid electrolyte member, one of the electrodes being exposed to the exhaust gas and the other being connected to the reference solid electrode through a lead-out wire extending in a radial direction within the solid electrolyte member. Such an oxygen sensing element is disclosed in Japanese Patent Publication (KOKAI) No. 9,497/1976. It is presumed that this oxygen sensing element does not possess the defects mentioned in the preceding paragraph. It is mentioned in that Japanese patent publication that the solid electrolyte member is formed about the reference solid electrode by chemical vapor deposition, ion plating, sintering or sputtering. However, the Japanese patent publication is silent on the particulars of the method of manufacturing the oxygen sensing element and, in actuality, some difficulties are encountered in the course of its manufacture. For example, when it is intended to manufacture the oxygen sensing element by sintering, it is difficult to obtain a molded product to be sintered. That is, in the step of press molding the finely divided solid electrolyte material, intrusion of the finely divided material into a minute gap between the lead-out wire and the groove on the mold for the lead-out wire cannot be completely avoided. Such intrusion makes it difficult to release the lead-out wire from the mold, and upon releasing, imposes a stress on the lead-out wire and on the portion of the molded product surrounding the lead-out wire. Due to the stress, this portion of the molded product is poor in sintering characteristics as compared with the remainder of the molded product. Thus, the molded product is liable to be cracked upon sintering, and the sintered product is not homogeneous and is poor in airtightness in the portion of the sintered product contacting the lead-out wire.